Upolu is an island in Samoa, formed by a massive basaltic shield volcano which rises from the seafloor of the western Pacific Ocean. The island is 75 kilometres (47 mi) long, 1,125 km2 (434 sq mi) in area, and is the second largest in geographic area as well as with 135,000 people the most populated of the Samoan Islands. Upolu is situated to the southeast of the "big island", Savai'i.